The present invention relates to absorbent articles such as diapers, incontinence protectors, sanitary napkins, wound dressings and similar articles which are applied against skin, and concerns methods aiming at the reduction of unwanted side-effects which sometimes occur during use of said articles.
1. Background
Absorbent articles of this kind are known in a number of different types. Conventionally, the absorbent body in these articles is produced by dry-defibration of cellulose pulp from, for instance, rolls, bales, or sheets and converting the pulp into a web of fluffed pulp sometimes with the admixture of what is known as superabsorbents, which are polymers with the ability to absorb several times their own weight of water or body fluids.
All uses of products which are applied against skin may lead to unwanted side-effects. These may occur as a result of occlusion, moisture, mechanical, microbial, and enzymatic factors which all, to different degrees, interact and amplify the influence of each other and may cause different forms of skin irritation and primary or secondary skin infections which sometimes occur in users of said articles. An increase in pH is a normal phenomenon during use of absorbent articles in contact with skin. However, several unwanted side-effects may occur as a result of, or in connection with, a pH-increase. One example of an unwanted side-effect of this kind is irritational contact dermatitis which exhibits a connection with the surface-pH of skin. Factors Predisposing Cutaneous Irritation, Wilhelm K-P et al, Dermatologic Clinics 8, No. 1, 1990.
Another example of unwanted effects is the activity of enzymes such as lipases and proteases which exhibit a strongly pH-dependent activity which increases with increasing pH. The skin starts to decompose and becomes sensitive to mechanical forces and bacterial attacks. Etiologic Factors in Diaper Dermatitis: The Role of Urine, Berg R. W. et al, Pediatric. Dermatology. 3, No. 2, 1986.
Another example of unwanted side-effects is that some bacteria such as Proteus can metabolise substances in urine and other body fluids and produce odorous substances such as ammonia and amines, which also raises the pH. At a high pH, the equilibrium for many odorous substances is shifted so that more volatile components are formed, resulting in a stronger odour than at a low pH.
An absorbent article such as a diaper, or the like provides an environment for micro-organisms which comprises access to moisture, nutrients, and heat. An unwanted side-effect of this is that growth of bacteria is promoted in such an environment. High numbers of bacteria constitute a risk of the appearance of infections. Moreover, a high presence of bacteria constitutes an increased risk for the creation of unpleasant odours caused by the formation of different substances, resulting from biological or chemical decomposition of components in body fluids such as, for instance, urine or menses.
Micro-organisms which may be associated with the occurrence of problems when using products in contact with skin may be of different types. Examples of micro-organisms which cause odour and those which constitute a risk for urinary tract infections are Proteus mirxc3xa1bilis, Proteus vulgaris, Echerichia coli, Enterococcus and Klebsiella.
Examples of micro-organisms which are associated with skin-infections and other skin problems are Candida albicans and Staphylococcus sp. and Streptococcus sp.
2. Related Art
It is known that a low pH is advantageous in order to reduce the occurrence of negative effects on skin.
Different ways of solving the described problems have been suggested. In the American patent U.S. Pat. No. 3,794,034 the importance of the pH in an absorbent article is described as well as the impregnation of an absorbent article with buffering substances which aid in keeping the pH in the article between 3.5 and 6.0, which is advantageous both for inhibiting the growth of unwanted bacteria and, in connection therewith, the creation of unwanted odours, and for avoiding negative effects on skin.
In the European patent 0,311,344 pH-control in an absorbent article is disclosed, wherein the buffering properties are obtained by using a partially neutralised superabsorbent material. In order to achieve sufficient inhibition of the growth of unwanted bacteria in the article, a separate bacteria inhibiting substance has also been added. The European patent EP 0,316,518 discloses buffering of pH in an absorbent article by using a polymeric organic acid.
One drawback in using bacteria suppressing agents, as described in EP 0,311,344, is that these agents are often selective and may be associated with risks, for instance in the form of allergenic properties or negative ecological consequences in garbage handling. Furthermore, the use of this type of agents may involve a risk that resistant strains of micro-organisms arise.
In EP 0,316,518 an absorbent article is disclosed, comprising a pH-controlling substance in the form of a partially neutralised superabsorbent material. It is stated that pH in the absorbent article is in the interval 5-6. However, this is not a sufficiently low pH in order to obtain a sufficient inhibition effect on the micro-organisms which are present since the majority of micro-organisms require a pH below 5 in order to be affected to a significant degree.
One difficulty when evaluating the influence of different components on the pH in absorbent articles of the aforementioned kind is that the degree of acidity in cellulose fluff pulp varies depending on the production method. Chemical cellulose pulp (CP) varies between pH 6-8.5 and chemical thermomechanical cellulose pulp (CTMP) varies between 5.5-8.5. Variations outside said intervals also occur.
The object of the present invention is to achieve an absorbent article of the kind mentioned in the introduction which makes it possible to use the article during a longer period of time without the appearance of undesired side effects such as unpleasant odour, increased risk of infections or negative skin effects which are due to the growth of micro-organisms resulting from an unwanted raise in skin-pH, or from other reasons.
An absorbent article of the kind mentioned in the introduction, wherein the problems connected with using the article for an extended period of time have been substantially removed, has been achieved through the invention with an absorbent body in the absorbent article comprising a pH-controlling substance in the form of a partially neutralised superabsorbent material and in that pH in the absorbent article after wetting is in the interval 3.5-4.9, whereby growth of unwanted strains of micro-organisms is restricted and the occurrence of unwanted side effects resulting from the use of the article is reduced.
It has been shown that if the absorbent body in an absorbent article comprises a pH-controlling substance in the form of a partially neutralised superabsorbent material which after wetting creates a pH in the absorbent article which is in the interval 3.5-4.9, and preferably 4.1-4.7, a significant inhibition effect on the growth of unwanted micro-organisms is obtained. The inhibition effect is based on the fact that many micro-organisms have an activity which is strongly pH-dependent and decreases with decreasing pH, which means that a decrease in pH leads to a decrease in activity in most micro-organisms which, in turn, leads to a decrease of bad smell as well as negative effects on skin in the form of skin-irritation and primary or secondary skin-infections and a generally lower risk of infections.
Enzymes, such as lipases and proteases have an activity which is strongly pH-dependent and decreases with decreasing pH, which means that a decrease in pH will also lead to a decrease in enzymatic activity and an accompanying reduction in negative skin effects.
An absorbent body containing absorbent material and absorbed fluid is a heterogeneous system from a pH point of view. The system may contain superabsorbent material, fibres and liquid containing several kinds of ions. In order to obtain reproducible pH-values, measurements must be made in several places in the absorbent body and the mean value calculated.
An absorbent body in accordance with the invention may also comprise other absorbent materials besides a partially neutralised superabsorbent material, for instance fluffed cellulose pulp. It has proven advantageous to use a partially neutralised superabsorbent material as above in combination with fluffed cellulose pulp having a pH below 7, preferably below 6 which gives a further improved effect.
A suitable fluffed cellulose pulp may consist of a chemical thermo-mechanical cellulose pulp having a pH=2.5-8.5, preferably 2.5-6.5 and most preferably 2.5-5.5, or of a chemical cellulose pulp having a pH=2.5-8.5, preferably 2.5-8.0 and most preferably 2.5-7.0.
In order to obtain a suitable degree of acidity in the cellulose pulp, its pH may be controlled during the production process, for instance by adding an acidifying substance. This added substance may, for instance, consist of SO2-water. In this way, it is also possible to neutralise NaOH which may occur in the pulp. Another way of achieving a suitable degree of acidity in the cellulose pulp is to add a suitable acid after production of the pulp.
A suitable, partially neutralised superabsorbent material may, for instance, consist of a cross-linked poly-acrylate of the kind which is described in the European patent EP 0 391 108 in the name of Cassella AG. Other types of superabsorbent material than the above indicated, and having corresponding properties, may be used. A suitable proportion of superabsorbent material in the article is 5-100%, preferably 15-50%, and most preferably 15-50%. The proportion of superabsorbent material which has been stated to be preferred relates to areas of the article which will absorb the major part of the fluid and, consequently, does not concern end-portions or edge-portions or similar parts of the article since such parts thereof mostly do not significantly contribute to the absorptive function of the article.
Examples of the relation between the degree of neutralisation and pH in the superabsorbent material are given below.
From the table it can be learned that the degree of neutralisation should normally be lower than 45% and preferably 35%. However, the degree of neutralisation should preferably be higher than approximately 20%.
An absorbent body in accordance with the invention, comprising a partially neutralised super-absorbent material in accordance with the invention may have somewhat lower absorption capacity when compared to a corresponding absorbent body containing a conventional super-absorbent material. Such a lowering of the absorption capacity may be compensated by increasing the amount of absorbent material to a corresponding degree.
An absorbent body in accordance with the invention, comprising a partially neutralised super-absorbent material may also comprise some type of conventional bacteria-inhibiting substance such as parabens or bensoic acid. Such bacteria inhibitors normally exhibit an increased effect at a lower pH.